The invention relates to a catalyst for oxidation of exhaust gas constituents, to a process for producing such a catalyst, to the are of an aforementioned catalyst, and to produced using such a catalyst.
Catalysts, especially for oxidation of exhaust gas constituents, have been known for some time and are used in many cases, for example connected downstream of internal combustion engines, to remove undesirable exhaust gas constituents from the exhaust gas of the internal combustion engines.
A key molecule in the reduction of the level of unwanted exhaust gas constituents here is NO2.
For minimization of carbon-containing fine particulates, what are called particulate separators or particulate filters are typically used in motor vehicles. A typical particulate separator arrangement in vehicles is known, for example, from EP 1 072 765 A2. Such particulate separators differ from the particulate filters in that the exhaust gas stream is conducted along the separation structures, whereas the exhaust gas must flow through the filter medium in particulate filters. As a result of this difference, particulate filters have a tendency to blockage, which increases an exhaust gas backpressure, i.e., causes an unwanted pressure increase at the exhaust gas outlet of an internal combustion engine, which in turn reduces the engine power and results in increased fuel consumption of the internal combustion engine. One example of such a particulate filter arrangement is known from EP 0 341 832 A2.
In both above-described arrangements, an oxidation catalyst arranged upstream of the particulate separator or particulate filter in each case oxidizes the nitrogen monoxide (NO) in the exhaust gas with the aid of residual oxygen (O2) likewise present to give nitrogen dioxide (NO2) according to the following equation:2NO+O2⇄2NO2  (1)
In this context, it should be noted that the equilibrium of the above reaction is to the side of NO at high temperatures. This in turn has the consequence that the achievable NO2 contents at high temperatures are limited because of this thermodynamic restriction.
The NO2 in turn reacts in the particulate filter with the carbon-containing ultrafine particulates to give CO, CO2, N2 and NO. With the aid of the strong oxidizing agent NO2, therein thus continuous removal of the fine particulates accumulated, and so it is possible to dispense with regeneration cycles such as those which have to be performed in a costly and inconvenient manner in other arrangements. In this context, reference is made to passive regeneration, according to the following equations:C+2NO2→2NO+CO2  (2)NO2+C→NO+CO  (3)
The formation of carbon monoxide according to equation (3) plays only a minor role; usually, complete oxidation of the carbon takes place up to the +4 oxidation state, in the form of carbon dioxide, this oxidation requiring two NO2 molecules per carbon molecule.
As well as NO2, SO3 is also formed over platinum-containing NO oxidation catalysts from sulphur present in the fuel and/or motor oil. The SO3 and NO2 condense at cold points in the exhaust tract to give highly corrosive sulphuric acid and nitric acid respectively, and so the exhaust system has to be constructed in stainless steel up to the particulate filters in order to avoid corrosion. A further problem with the oxidation of SO2 is a deactivation of the NO oxidation catalyst by sulphates, and in the worst case by the physisorption of sulphuric acid at the catalyst surface.SO3+H2O→H2SO4 
The deactivation of the catalyst can be reversed in the typically Al2O3-based catalysts according to the prior art by raising the exhaust gas temperature to more than 500° C., but these high exhaust gas temperatures barely ever occur in modern, use-optimized internal combustion engines.
If the carbon deposited in the particulate filter is not fully oxidized with the aid of NO2, the carbon content and hence the exhaust gas backpressure rise constantly. In order to avoid this, the particulate filters are currently increasingly being provided with a catalytic coating for oxidation of NO (EP 0 341 832 A2). This involves catalysts with a platinum-containing coating. The disadvantage of this known process, however, is that the NO2 formed over the particulate filter can serve only to oxidize particles which have been deposited downstream of the catalytically active layer for NO oxidation, thus meaning within the filter medium. If, in contrast, a layer of deposited particles, called a filtercake, forms on the filter surface and hence on the catalytically active layer, the NO oxidation catalyst on the particulate filter side is downstream of the filtercake, and so the soot particles deposited there cannot be oxidized with the aid of NO2 from the NO oxidation catalyst applied to the particulate filter. In addition, strictly speaking, only the catalyst layer applied to the dirty gas side contributes to the performance of the system, since the NO2 formed catalytically on the clean gas side cannot come into contact again with the soot deposited on the dirty gas side and within the filter material.
A further problem with the coating of the particulate filter is that the geometric surface areas of the filter are much lower than those of the catalytic converter substrates typically used. The reason for this is that the filters require relatively large free cross sections and hence free volume on the dirty gas side for incorporation of soot and motor oil ash. If ceramic filter substrates are used, this is achieved by a low cell density of 50 cells per square inch (cpsi) to 200 cpsi. Compared to this, pure catalytic converters typically have cell densities of 400 cpsi to 900 cpsi. The rise from 50 cpsi to 900 cpsi results in an increase in the geometric surface area from 1 m2/l to 4 m2/l, which enables considerable increases in conversion in the catalytic converters.
For these reasons, in spite of the catalytic coating of the filter, it is not possible to dispense with an NO oxidation catalyst upstream of the particulate filter, resulting in a relatively large installation volume. This is the case even when the NO oxidation catalysts and the particulate filters form one installed unit in which the inlet region of the particulate filter takes the form of an NO oxidation catalyst, as described, for example, in DE 103 270 30 A1.
In all these variants for passive regeneration by means of NO2, it should be noted that the soot oxidation cannot be enhanced further below 200° C. to 230° C. even by a rise in the amounts of NO2. At about 370° C., the conversion maximum is attained. From this temperature, the soot oxidation proceeds according to the above-described reaction (2), meaning that two NO2 molecules react with one carbon molecule. In terms of mass, this means that 0.13 g of carbon can be oxidized with one gram of NO2; in other words, soot oxidation can be increased as desired by raising the amount of NO2.
If the temperatures are below 200° C. to 230° C., reliable function of the particulate filter thus cannot be ensured. This typically occurs in the case of engines having to stress levels and installed in motor vehicles, for example in passenger vehicles, urban buses or refuse vehicles, which have additional high proportions of idling time. Therefore, specifically in such cases, a second means of particulate filter regeneration is employed, in which exhaust gas temperature is actively raised. This is typically done by the addition of hydrocarbons (HC) upstream of catalysts, especially upstream of HC oxidation catalysts. Because of the exothermicity of the oxidation of the hydrocarbons added with the aid of the catalysts, a distinct temperature rise is achieved:“HC”+O2→CO+H2O  (4)“HC”+O2→CO2+HO  (5)
In order to sufficiently thermally stabilize these catalysts, they usually contain palladium as an active component. Although palladium knows a very good HC oxidation activity on, it does not have any NO oxidation activity and additionally reduces the NO oxidation activity of any platinum present in the catalysts. As a result of this, HC oxidation catalysts have a much lower NO oxidation activity than pure NO oxidation catalysts.
If the metered addition of hydrocarbons achieves a temperature rise to more than 600° C., the carbon deposited in the particulate filter is oxidized or burnt off with the aid of oxygen, according to the following equations:C+O2→CO2  (5)2C+O2→2CO  (7)
However, in the case of this so-called active filter regeneration, there is the risk that the exothermic burnoff of the carbon-containing soot can result in a significant temperature rise to up to 1000° C. and hence in damage to the particulate filter, and/or downstream catalysts. Since the temperature increase additionally has to be maintained for several minutes in order to ensure quantitative oxidation of the soot particles, the demand for hydrocarbons is not inconsiderable and reduces the efficiency of the internal combustion engine, since the fuel is typically used as the hydrocarbon source.
A further problem, in contrast to passive regeneration, is the high carbon monoxide emissions during the regeneration, the formation of which is described in equation (7). For this reason a further catalyst for oxidation of the carbon monoxide formed during the regeneration has to be positioned on the particulate filter and/or downstream of the particulate filter in order to avoid the emission thereof into the environment.
A simple combination of passive and active regeneration by adding hydrocarbons upstream of NO oxidation catalysts is not productive:
As a result of the temperature rise to more than 600° C., barely any further NO2 is formed over the NO oxidation catalysts because of the thermodynamic restriction. In addition, the oxidation of NO is hindered by the large amounts of hydrocarbons, as a result of which there is a significant reduction in NO2 formation. The effect of this is that the particles have to be oxidized with the aid of oxygen alone, since no NO2 is available in this phase, which prolongs the regeneration time and leads to high carbon monoxide emissions.
At the area time, the NO oxidation catalysts are much less stable to thermal damage than catalysts for hydrocarbon oxidation, since there irreversible sintering of the active components and hence a decline in the NO oxidation activity at temperatures exceeding 550° C.
As well as the oxidation of carbon-containing particulates in particulate filters, NO2 is also used to accelerate the SCR reaction (selective catalytic reduction reaction), or in the case of NOx storage catalysts for formation of nitrates.
As already indicated above, a problem with the NO oxidation catalysts is that they are deactivated in the presence of sulphur oxide which is formed by the combustion of sulphur from fuel or lubricant oil. One means of alleviating this problem is to use titanium dioxide rather than Al2O3, which is typically used as the catalyst support material: sulphur trioxide or sulphuric acid is adsorbed to a much lesser degree thereon; at the same time, desorption is possible at much lower temperatures. However, in the case of use of TiO2, the problem arises that conversion from the anatase form to the rutile form takes place at relatively high temperatures, which results in a decline in the BET surface area (to DIN ISO 9277: 2003-05) and an accompanying decline in activity.
A further disadvantage of processes currently used is that such known catalysts are produced by wet-chemical means, the result of which is chat the actual active platinum component is often surrounded by Al2O3 support material and is thus inaccessible to the actual reaction.